The present invention relates to a steering method and steering system for a tracklaying vehicle which has a pair of brakes for the right and left output shafts thereof and in which the driving force of the hydraulic motor is transmitted to each of the output shafts through the differential steering means.
There is known a steering system for a tracklaying vehicle such as bulldozers, in which a hydraulic motor is driven by a hydraulic pump actuated by an engine and the output of the hydraulic motor is transmitted to right and left output shafts respectively through differential steering means such as planetary gear mechanisms so that the rotational speeds of the right and left sprockets are made different from each other thereby to allow a turn of the vehicle.
Such a steering system for a tracklaying vehicle has the drawback that since the hydraulic motor receives the reactive force of the turn resistance of the vehicle during a turn of the vehicle and since a greater reactive force acts on the hydraulic motor as the turn radius of the vehicle becomes smaller, a turn radius exceeding the capacities of the hydraulic pump and the hydraulic motor cannot be obtained. In addition, since a steering system driven by a hydraulic motor is designed to gain all of the power required for turning from hydraulic pressure, the vehicle must have sufficient engine power.
To solve the above problems, Japanese Patent Publication (KOKAI) Gazette No. 63-235173, for example, proposes a system in which right and left steering brake valves are provided in a closed circuit for the hydraulic pump and the hydraulic motor and in which a pair of brakes are provided on a transverse shaft, the system being characterized in that when the operational position of the steering lever exceeds the position corresponding to the intermediate operational amount, the output of the hydraulic motor is made to be zero while either of the right and left sprockets being braked so that a reactive force does not act on the hydraulic motor when the vehicle make a turn with a small turn radius such as a pivot turn.
The prior art technique disclosed in the above publication however reveals the following drawback. Regarding the relationship between the timing of braking the sprocket by the brake and the timing of disconnecting the hydraulic motor from the driving force, if the former (the braking timing) is slower than the latter (the disconnecting timing), the relative speed of the right and left sprockets is dependent on only the load of the road surface imposed on each crawler belt so that the turn radius of the vehicle body cannot be constant. On the other hand, if the former (the braking timing) is faster than the latter (the disconnecting timing), the vehicle body receives a great impact at the moment the vehicle starts a pivot turn.
The present invention is directed to overcoming the foregoing problems and a prime object of the invention is therefore to provide a steering method and steering system for a tracklaying vehicle, which enable a minimum turn radius no less than the discharging capacities of the hydraulic motor and the hydraulic pump and positively prevent the unstable traveling and turning behavior of the vehicle particularly when the vehicle makes a pivot turn.
In accomplishing the above prime object, there has been provided, in accordance with a first aspect of the invention, a steering method for a tracklaying vehicle provided with a pair of brakes for right and left output shafts and designed to allow the driving force of a hydraulic motor to be transmitted to each of the output shafts through differential steering means,
Wherein, during a pivot turn of a vehicle, an operation of the brake for either one of the output shafts and a transmission of the driving force from the hydraulic motor to the differential steering means are simultaneously carried out for a specified period of time and thereafter, the transmission of the driving force from the hydraulic motor to the differential steering means is interrupted.
In the invention, during a pivot turn of the vehicle, an operation of the brake for either one of the right and left output shafts and a transmission of a driving force from the hydraulic motor to the differential steering means are simultaneously carried out for a specified period of time and then, the transmission of the driving force from the hydraulic motor to the differential steering means is interrupted so that the hydraulic motor is brought into a free condition. With this arrangement, the timing of braking operation is delayed which makes it possible to avoid an undesirable situation caused by the free condition of the hydraulic motor continuing until actuation of the brake, that is, an undesirable situation where the relative speed of the right and left sprockets is dependent upon only the load of the road surface imposed on the crawler belts. In addition, this arrangement also avoids such an unfavorable situation that a great impact is imposed on the vehicle body at the moment the vehicle starts a pivot turn. Since the driving force transmitted from the hydraulic motor to the differential steering means is shut off during a pivot turn, the reactive force of the road surface does not act on the hydraulic motor. Therefore, a pivot turn can be carried out with a conventional hydraulic motor having small capacity.
According to a second aspect of the invention, there is provided a steering system for a tracklaying vehicle provided with a pair of brakes for right and left output shafts and designed to allow the driving force of a hydraulic motor to be transmitted to each of the output shafts through differential steering means, the steering system comprising:
(a) brake operating means for independently operating the right and left brakes;
(b) driving force transmission interrupting means for interrupting a transmission of the driving force from the hydraulic motor to the differential steering means;
(c) pivot turn control command signal detecting means for detecting an issue of a vehicle pivot turn control command signal; and
(d) controlling means for controlling the brake operating means and the driving force transmission interrupting means, in response to an output from the pivot turn control command signal detecting means, such that after an operation of the brake for either one of the right and left output shafts by the brake operating means and a transmission of the driving force from the hydraulic motor to the differential steering means have been simultaneously carried out for a specified period of time, the transmission of the driving force from the hydraulic motor to the differential steering means is interrupted by the driving force transmission interrupting means.
The second aspect of the invention is associated with a system for giving a concrete form to the steering method for a tracklaying vehicle of the first aspect of the invention and exerts the same effect as that of the first aspect of the invention.
Preferably, the steering system according to the second aspect of the invention further comprises engine rotational speed detecting means for detecting the rotational speed of an engine, and the controlling means controls the engine in such a way that the rotational speed of the engine detected by the engine rotational speed detecting means during a pivot turn of the vehicle is no more than a specified rotational speed value. This arrangement avoids such an unfavorable situation that while either one of the brakes being operated to stop its corresponding drive shaft, the hydraulic motor receives a driving force from the other drive shaft being rotated, resulting in an overrun of the rotation of the motor. As a result, the service life of the hydraulic motor can be increased.
The controlling means preferably executes desired control after an elapse of a specified time after receipt of an output from the pivot turn control command signal detecting means. With this arrangement, the operator""s intention can be confirmed before executing the control for a pivot turn so tat safety can be ensured even if a mistake is made in operating the system.
The pivot turn control command signal detecting means preferably detects an issue of a vehicle pivot turn control command signal when a pivot turn control command switch provided in a turn operation lever has been depressed while the stroke of the turn operation lever is in a pivot turn region. With the provision of such a pivot turn control command switch, final confirmation can be made for the sake of security as to whether the operator has an intention to execute a pivot turn, even when the stroke of the pivot turn operation lever is in the pivot turn region.
Herein, the pivot turn region is in the vicinity of the lever stroke end of the turn operation lever and occupies no more than 20% of the entire lever stroke. This facilitates the adjustment of the turn operation lever within the pivot turn region.
Preferably, the pivot turn control command signal detecting means detects an issue of a vehicle pivot turn control command signal by detecting that the turn operation lever has reached the lever stroke end. With this arrangement, the power turn region of the steering control by use of the hydraulic motor can be utilized as much as possible.
According to the second aspect of the invention, it is preferable to provide a mode selection switch for selecting whether or not the control by the controlling means is to be executed. Use of the mode selection switch has the following effect: when it is determined that the operator has no intention of executing a pivot turn operation, the mode selection switch is turned ON, whereby the automatic pivot turn operation is interrupted so that control can be performed in accordance with the operator""s intention.
Preferably, the driving force transmission interrupting means shuts off the transmission of the driving force from the hydraulic motor to the differential steering means by opening a communication valve disposed in a communication path which communicates the upstream and downstream of the hydraulic motor with each other within a hydraulic drive closed circuit including a hydraulic pump and a hydraulic motor. By employing such a hydraulic circuit configuration, the desired object can be accomplished with an extremely simple configuration.
The driving force transmission interrupting means may be a motor clutch for shutting off the transmission of the driving force from the hydraulic motor to the differential steering means.